In order to meet environmental regulation in many jurisdictions, there is an increased interest in using spray guns which atomize the paint with a high volume flow of low pressure air. Such guns produce less overspray than spray guns which use a higher velocity high pressure air for achieving atomization. An HVLP paint spray gun is generally defined as a spray gun using air at no greater than 10 psig (0.7 Kg/cm.sup.2) at the nozzle for atomization and for pattern shaping. In California, for example, if the spray gun operates with no more than 10 psig at the nozzle, it is exempt from licensing requirements.
There are two general types of HVLP paint spray guns. In one type, an air turbine is used to supply the HVLP air through a relatively large diameter hose to the gun. The large hose can interfere with use of the gun. In a second type of HVLP spray gun, high pressure air is supplied to the gun from a conventional source, such as an air compressor or a compressed air line in a factory. The spray gun has one or more internal orifices for dropping the high air pressure to a low pressure of no more than 10 psig. Difficulty can occur in maintaining the maximum 10 psig pressure in the gun when air flow requirements change. The spray gun requires low pressure air both for atomization and for pattern shaping. During spraying, the atomization air does not change. However, the operator can adjust the flow of pattern shaping air to produce a round pattern where no pattern shaping air is required or a long narrow fan shaped pattern, where a maximum flow of pattern shaping air is required at a pattern between the two extremes. If a single orifice is used to drop the air pressure for both the atomization air and the pattern shaping air, changing the flow of pattern shaping air will result in a change in the pressure drop across the orifice. Consequently, selecting the orifice to produce the maximum 10 psig atomization air when there is no pattern shaping air flow will result in a drop in the atomization air pressure when pattern air flow is increased. The drop in atomization air pressure can adversely affect the quality of the paint atomization. In one improved design HVLP spray gun operated from a source of high pressure air, different fixed orifices are used to drop high air pressure air to a maximum of 10 psig for atomization and for pattern shaping. By using different orifices, a change in the flow of pattern shaping air has little adverse affect on the pressure of the atomization air. However, the orifices have been fixed in the spray gun body. The fixed orifices are calibrated for a particular size fluid tip which discharges the paint and for a particular size air cap which discharges the atomization air and the pattern shaping air. In another type of HVLP spray gun, a pattern shaping air control valve and an atomization air control valve are operated together to maintain a constant atomization air pressure as the pattern shaping air flow is adjusted. These valves also are designed for a particular fluid tip/air cap configuration and are not changeable by the gun operator. Consequently, the prior art HVLP spray guns are suitable for applying only a selected group of paints having flow characteristics matching the design of the fluid tip and the air cap. It has been necessary to have different HVLP spray guns available for applying paints with substantially different flow properties.
Baffles have been used in HVLP spray guns of the type operated from a high pressure air source. The high pressure air is dropped to a low pressure by fixed orifices in the gun body. A baffle has been used to split atomization and shaping air at the front of the spray gun and also to uniformly distribute the low pressure air to pattern shaping orifices on opposite sides of the air cap and to the atomization orifice. Typically, the baffle is designed for minimum pressure drop. Since low pressure air is supplied to the baffle, there may be an uneven pressure drop through the baffle.